According to Danish researchers being obese can hinder conception and this is especially true if both partners are overweight.
The research adds more fuel to the on-going debate over obesity and it's health implications.
The researchers at Aarhus University Hospital in Denmark, used data from the Danish National Birth Cohort, a nationwide study of pregnant women and their offspring, which enrolled more than 100,000 women between 1996 and 2002.
They tracked 48,000 Danish couples between 1996 and 2002, including about 7,600 couples where both the man and woman were either overweight or obese according to standards set by the World Health Organization.
When they calculated how long it took couples to conceive a baby once they began unprotected sex, they found that where both the man and woman were obese, it took three times as long to conceive than for couples of normal weight.
It is already known that excess weight can affect fertility in women and men as individuals, but this study looked at the fertility of couples who were both overweight.
Sub-fertility is normally defined as a waiting time to pregnancy of more than 12 months from the time a couple starts to have unprotected sex with an intention to conceive.
Other research has also shown that semen quality and levels of reproductive hormones diminish in overweight men, and that being overweight can also harm ovulation, conception and early fetal development in women.
Cecilia Ramlau-Hansen, who led the study, warns that if further research supports the findings of the study it could have major implications for population levels, particularly in parts of the world where obesity and low fertility were, or would become, more common.